Question

pH and Buffers IV. pH of a Buffer Solution Mass of sodium acetate, CH,COONa, g 4.23g Calculated pH Measured pH buffer solution prepared with dissolved CH.COONa + 8.5 mL CH,COOH(aq) 13.84 40 ml buffer + 1.0 mL of 6.0 M HCl(aq) 1.30 40 ml. buffer + 1.0 mL of 6.0 M NaOH(aq) 5.90 Measured pH Calculated pH deionized water 5.78 1.55 40 mL DI water + 1.0 mL of 6.0 M HCl(aq) 40 mL DI water + 1.0 mL of 6.0 M NaOH(aq) 11.40 Compare the change in pH you observed when 1.0 mL of 6.0 M HCl is added to 40 mL of the buffer versus 40 mL of water. Is there a difference, and if so, why? Do the same for your observation of the change in pH of the buffer versus the water upon the addition of 1.0 mL of 6.0 M NaOH solution. REPORT SHEET 127

thats all i'm given

would it be the Ka1 which js 1.8x10^-5

Answer 1

Part IV.

i) a) Moles of CH3COONa = 4.23 g/(82 g/mol) = 0.0515854 mol

Moles of CH3COOH = (8.5 mL * 1.05 g/mL)/(60 g/mol) = 0.14875 mol

According to the Henderson-Hasselbulch equation:

pH = pKa + Log([CH3COONa]/[CH3COOH])

i.e. Calculate pH for the buffer = 4.74 + Log(0.0515854/0.14875) = 4.28

b) calculated pH after adding HCl to buffer = 4.74 + Log{(0.0515854-0.006)/(0.14875+0.006)} = 4.21

c) calculated pH after adding NaOH to buffer = 4.74 + Log{(0.0515854+0.006)/(0.14875-0.006)} = 4.35

ii) a) Calculate pH for the DI water = -Log[H⁺] = -Log(10^-7) = 7

b) calculated pH after adding HCl to DI water = -Log(1*6/41) = 0.835

c) calculated pH after adding NaOH to DI water = 14-{-Log(1*6/41)} = 13.165

The notable thing is that the change in pH after the addition of HCl solution and NaOH solution for the buffer can be neglected.

Explanation: A buffer solution resists the change in pH by the addition of a small amount of strong acid or strong base.

Whereas the change in pH after the addition of the HCl solution and NaOH solution for DI water is much more significant.

Explanation: DI water as such is neutral (pH = 7); when you add strong acid to it, it will be acidic (pH < 7), when you add a strong base to it, it will be basic (pH > 7)